There have been provided heretofore devices for opening synthetic-resin foil bags to enable their filling at an appropriate station along a transport path of a carrier for such bags. Synthetic-resin foil bags, of the type with which the present invention is concerned, are generally supplied from the bag-fabrication machinery in the form of stacks of flattened bags. The bags may be simple sacks of rectangular outline and formed with three rectolinear sides in which the synthetic-resin foils are thermally fused together along seams. Alternatively, they may be bags of the type commercially available under the designation "DOYPACK" of the type described in U.S. Pats. Nos. 3,380,646, 3,637,133 and 3,935,993 and in which a pair of substantially rectangular thermoplastic panels are formed at their bottoms with gussets or a bottom panel of substantially inverted-V cross section so that the section of the bottom has a W-cross section. The insert panel is thermally welded to the main wall panels over a curvilinear seam or straight seams having the configuration of a trapezoid. The insert panel permits the bottom of the bag to be spread apart and permits the bag to be essentially self-standing, when filled.
There are two main systems conventially used for opening such bags.
In a first such system, the bags are withdrawn from a magazine or stack, in which they are stored flat, by a suction member and are then introduced into the pincers of a support mounted on a transfer conveyor. At a station downstream from the opening device the bags are filled. The opening device, downstream from the magazine and upstream from the filling station, is provided with vacuum ramps along which the bag is carried and which spread the lips of the open end of the bag. Air at low pressure is blown into the spread mouth of the bag to inflate the latter.
In a second system, the bag-opening arrangement comprises a vacuum means for spreading the lips of the mouth of the bag and a mechanical element which is introduced through the open end to spread apart the walls of the bag to the bottom.
These two systems have significant disadvantages in practice.
In the case of the first system, especially when thin-foil bags are to be opened and the opening is difficult, an elevated pressure is developed within the bag which is propagated towards the bottom quite vigorously to generate a shock wave which is able to rupture the foil of the bag. Furthermore, for certain types of bags it is almost impossible to effect their opening by inflation and it is possible to note a substantial percentage of unopened bags on the machine.
In the second type of bag-opening apparatus, the introduction of a mechanical member into the fragile bag is capable of causing deterioration of the film from which it is constituted. In addition, the deep introduction of a mechanical element into a bag which is being displaced horizontally relatively rapidly is difficult to synchronize with the bag-translation device. As a consequence it is necessary to reduce the speed of translation and hence the rate at which bags can be supplied to a filling station. Mechanical engagement of the opening member with the walls of the bag is liable to cause damage thereto.
Both of the prior-art devices described above are characterized by the need to provide the bag-opening device downstream from the magazine and upstream from the filling station. This requirement for a separate bag-opening station between the magazine and the filling station increases the size of the machine and makes it necessary to provide a relatively large apparatus for opening and filling such bags.